1. Field
The present disclosure relates to optical communication equipment and, more specifically but not exclusively, to coherent detection of modulated optical signals.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Coherent detection is used in optical transport systems, for example, because it is compatible with advanced modulation formats, such as quadrature phase-shift keying (QPSK) and quadrature-amplitude modulation (QAM), which enable the corresponding system to achieve a relatively high spectral efficiency, e.g., better than 1 bit/s/Hz. The use of polarization multiplexing serves to further increase the bit rate and spectral efficiency approximately by a factor of two. However, a major challenge of this particular transmission format is the need for fast and precise polarization tracking and, in the case of homodyne detection, for phase and carrier-frequency locking of the optical local-oscillator signal.